The present invention relates to a picture reading apparatus which, by projecting an original picture on a photoelectrical element(s), reads out picture information of the original picture, and particularly relates to a device with improved resolution which operates by reading out information of an original picture by scanning the original picture in line sequence by using one or more line sensors composed of a plurality of linearly aligned photoelectric elements.
In order to improve the resolving power of a picture reading apparatus which uses a line sensor(s), it is known to increase the number of photoelectric elements composing the line sensor. However, the maximum number of photoelectric elements is limited. Accordingly there have been proposed various kinds of means for obtaining better resolving power by using a line sensor having relatively small number of photoelectric elements.
For example, the arrangement shown in FIG. 12 is composed of a plurality of sets of projecting optical systems and a plurality of line sensors. The arrangement provides three similar lenses 17, 18 and 19 disposed in parallel with the surface of an original picture 16. An image of the original picture 16 is divided into three portions and each of the divided picture images is projected to focus on each of line sensors 20, 21 and 22, each of which outputs a respective picture image signal. In this case each of the sensors divides desired whole area of the original picture 16 to three equal areas. Then, onto the line sensor 20 images of from an A area to that of B of the original picture are projected, onto the one sensor 21 images of the area B to an area C of the original picture 16 are projected, and onto the one sensor 22 images of the C to an area D are projected. Accordingly, resolving power is improved up to three times as compared to the case in which the whole area from the A to the D is projected onto a single line sensor.
In addition, it has also been proposed to use only one projecting optical system, an optical path being separated to two optical paths by using a half mirror to divide the area of the original picture into two equal parts, and then projecting each of the divided areas onto the respective lens sensors. The latter method produces excellent resolving power, nearly two times than that of the usual case.
FIG. 13 is an example in which a projecting lens 24 and a plurality of line sensors 26, 27 and 28 are used. Between the projecting lens 24 and each of the sensors a half prism 25 is disposed to divide a light flux transmitted by the lens 24, and to emit each of the divided light fluxes to a respective line sensor. In this case too the entire area of the original picture 23 is divided into three equal parts, and each of areas AB, BC or CD is projected onto a respective line sensor. In this case resolving power three times better than the usual method is obtained.
Further, there has also been proposed an arrangement as shown in FIG. 14. In this arrangement, at the rear stage of a lens 30, a dual-faced mirror 31 is provided to divide the light flux into two light fluxes, and to emit the light fluxes to line sensors 32 and 33, respectively. In this arrangement a desired area of the original picture 29 is divided into two parts, AB and BC to improve resolving power up to two times. In addition, as a means for improving resolving power without dividing light flux there has been known a device, as shown in FIG. 16, which comprises a plurality of line sensors 34 and 35 which are disposed in parallel and offset lengthwise from one another such as to be phase shifted from one another. In this device the images of the linear area of the original picture are shifted to a direction of the line sensors being disposed in parallel, and picture information in the identical area is read out with a plurality of the line sensors. In the device each of the several unit elements is disposed so that respective phases are offset from one another, which results in that each of output information of the line sensors is mutually interpolated. As can be judged from the above, the total resolving power is improved based on the number of line sensors being used.
Each of the devices according to the above described prior art has practical disadvantages. In the means shown in FIG. 12 in which a plurality sets of lenses and line sensors are used, it is extremely difficult to provide a desired number of lenses all of which have identical focal lengths, even if the lenses are of the same standard, for there is unavoidable unevenness in focal length of the lenses due to manufacturing tolerances. Accordingly, in practice lenses having relatively close focal lengths are used. In that case, as a distance between the lenses and the surfaces of the original picture, a value like a G.C.D. (the greatest common divisor) of each of values determined by biasing on each of focal lengths of respective lenses according to desired magnification is to be selected, so that an out of focusing phenomenon is apt to occur.
Next, the device shown in FIG. 13 uses only a single lens, so that the disadvantage described above is avoided. However, between the lens and each of the line sensors there is provided a half prism 25, so that efficiency of the optical system is restricted with aberration of the half prism 25, and further in order to take in, that is, emit a light flux to the line sensors radiated from the whole desired areas of the original picture, it is required that a considerably larger size of the half prism 25 be selected, which results in manufacturing difficulty and prohibitive expense.
There is also the idea to use a flat half mirror instead of using the half prism. However, the flat half mirror approach generates a difference in lenghts of the light path between the reflecting surface of the half mirror and the focusing plane and that of between the transmitting surface of the half mirror and the focusing plane, so that correcting means for correcting the difference is required. For example, one solution is to neglect to a certain extent slippage of focusing, the line sensors being disposed at positions where the identical magnification can be obtained. Or, parallel flat plane glasses are disposed at desired positions in the light path divided by the half mirror to correct the difference. In addition, where a thin half mirror is used, the opaque film is provided, appropriately coated with transparent film for handling the difference in length between the light paths, which lowers its flatness in response to increase of the size thereof. This results in occurrence of distortion in picture images, and further there are other disadvantages which cause flaws and maintenance difficulties.
In the prior device shown in FIG. 14 there is a problem of partial unevenness in the obtained light quantity. With respect to this problem Japanese Patent Application No. 60-73582 (entitled "AN APPARATUS FOR CORRECTING UNEVENNESS OF LIGHT QUANTITY IN AN OPTICAL REPRODUCTION SYSTEM" by the inventor herein) describes an arrangement wherein light emitted to two line sensors 32 and 33 such that light flux from the B point on the original picture 29 positioned on the optical axis of the lines 30 is divided into two light fluxes, and emitted to B' or B" on the two line sensors. On the other hand other light fluxes from the ends A or C of the original picture are the whole light quantity being transmitted by the lens or nearly same light quantity having emitted to an A' or a C', so that the quantity of light received by the line sensors has the distribution shown in FIG. 15. Further, the device shown in FIG. 16 must be adapted such that in order to project the identical picture image area sequentially by a plurality of line sensors, a picture image signal output from the preceding line sensor is stored in memory means and synchronized with a picture image signal output from the following sensor, and has to be read out and recombined with the image signal of the second line sensor.